1. Field of the Invention
The present invention relates to a system and method for determining the location of a towed marine object relative to the towing vessel. More particularly, the present invention relates to a system and method for determining the horizontal profile of a towed seismic marine cable.
2. Related Art
The search for hydrocarbons is being widely pursed, including water covered areas of high potential. Much of the water covered area is unexplored and little is known of the structural configuration of the covered formation.
In conducting marine seismic exploration, a long seismic cable (called a streamer) is conventionally towed behind a vessel. These cables generally carry detector apparatus such as seismic transducers or hydrophones which detect reflected wave energy and provided signals representative thereof, from a sound source such as dynamite which introduces an acoustical wave front into the earth formation underlying the body of water being explored. The hydrophones pick up seismic waves reflected from the geological structures in the underlying formation.
The cables are typically several thousand feet long, e.g., up to several miles and comprise sections of cable coupled together end to end. Usually each section will carry one or more seismic transducer or hydrophone devices. The cables are usually constructed having a neutral buoyancy, and various systems have been developed for determining the vertical profile of the cable, e.g., U.S. Pat. No. 3,439,319.
A problem which has not been overcome, although different approaches have been made, is the configuration of the streamer in the water as it is towed, i.e., the horizontal profile of the cable. Currents, wake and the like, cause the cable to move over an unpredictable path over the geological formation being surveyed. Thus the location of any given seismic device at the time it receives a reflected sound is not accurately known.
The general practice is to assume the cable lies in a straight line behind the towing vessel. This is probably rarely the case. To compensate for this inaccuracy, three dimensional surveys are sometimes employed. Such a system is shown in U.S. Pat. No. 3,906,352.
One system for attempting to locate horizontal angular position of a towed seismic cable is described in U.S. Pat. No. 3,953,827, which measures the angle between the tangent to the cable and a measuring device and a fixed and known direction, using an optimized curve of the cable and the space between measuring devices, to obtain the position in space of the measuring devices and interpolating the location of the seismic device along the curve. The system lacks accuracy since it is based on assumptions as to the horizontal profile of the cable.
It is an advantage of the present invention that the location of a towed marine object relative to the towing vessel can be accurately determined. It is a particular advantage that an accurate profile of the horizontal configuration of a towed seismic cable can be determined. It is a further advantage of the present invention that a method of continuous profiling of the seismic cable is provided.